A long-held idea that a relatively thin crust covers a molten interior, and that downward pressure of this crust due to earth contraction causes molten rocks to be forced out, has been too thoroughly disproved to now be at all seriously entertained. The fact that near-by volcanoes commonly erupt entirely independently, as in the case of Mauna Loa and Kilauea, shows that there can be no universal liquid beneath a relatively thin crust. Other arguments against liquidity of the earth's interior are that the earth acts like a body nearly as rigid as steel against the powerful tide-producing forces, and that earthquake waves which pass through the earth to a depth of at least 2,000 miles are the kind which require a solid medium for transmission. Let us then briefly consider more plausible views regarding the cause of volcanic action. First of all we may be sure that the earth is highly heated inside. Measurements in many deep borings show that the temperature increases at the rate of about 1 degree F. for each 50 to 60 feet downward, to depths greater than a mile. Accordingly, on the basis of 1 degree rise in 50 feet, at depths of 20 to 35 miles, the temperature must be great enough (2,120 degrees to 3,590 degrees F.), to cause all ordinary rocks to melt _if they were at the surface_. At such depths, however, the downward pressure upon the rocks is so great that their melting points are notably raised, and there is every reason to believe that under ordinary conditions the rocks 20 to 35 miles down are not molten. If we adhere to the older (nebular) hypothesis of earth origin, the interior heat of the earth is left over from the cooling, once molten, earth. On the basis of another (planetesimal) hypothesis, the earth's heat is due to the steady, powerful action of gravity causing the earth to contract. In any case, the earth is hot inside as proved by deep well records and igneous phenomena in general, and it is a contracting or shrinking body as proved by the many large scale zones of wrinkling or folding of rocks. If, then, highly heated solid rocks at reasonable distances down in any part of the earth are subjected to relief of pressure by an earth movement such as upward crumpling of the crust, or by readjustment of large fault blocks, such heated solid rocks would become molten. The very earth movement which brings about relief of pressure and melting may very reasonably be regarded as the power which forces some of the newly formed molten material higher up into the earth's crust, and even out upon the surface. This view harmonizes with the well-known fact, already mentioned, that the main belts of active volcanoes are also the main belts of active earth movements, such as earthquakes.